Implement yank-last-arg/yank-nth-arg

Support for yank-last-arg and yank-nth-arg has been added.
Combined with digit-argument, these functions provide a nice way
to copy arguments from previous commands fairly easily.

For example, to copy the 2nd argument from a command 2 entries back,
you can use: Alt+2 Alt+. Alt+.

I also tweaked digit-argument slightly so that when you start with
a negative argument (Alt--) the first digit can override the -1.
In Bash, to enter something like -2, you have to think backwards
and type Alt-2 Alt-- because Alt-- Alt-2 ends up giving you -12.